Biomedical electrodes are used in a variety of medical and veterinary applications and are configured to operate according to the amplitude, duration, type and direction of the current flowing into or out of the body of the subject. In monopolar electrosurgery, as in all situations where electrical current is flowing, a complete circuit must be provided to and from the current source. For example, a current that enters the body at the location where the electrosurgical procedure is being performed leaves it in another place and returns to the electrical generator. It is clear that when current of enough intensity to deliberately cut, ablate, heat or stimulate is brought into contact with the body of a subject in one location, great care must be taken to ensure that unintentional damage is not done to the subject at the place where the current is leaving the body. An electrode attached to the subject's body performs the task of collecting the current safely. This electrode is supposed to perform this task by providing a large surface area through which the current can pass. When the collected current is spread over a large area, the current density is low enough so as to render the process harmless to the subject. This electrode is often referred to as a dispersive electrode, return electrode, electrosurgical pad, electrosurgical plate, grounding pad subject plate, subject return electrode or Bovie plate. These return electrodes are used in many medical procedures, including, but not limited to, monopolar electrosurgery, arthroscopy, urology, gynecology, laproscopy, open surgery, cardiac defibrillation, heating and many others. The electrodes are available commercially from such vendors as ConMed Corp., Valleylab (div of Tyco), Minnesota Mining and Manufacturing (3M), Erbe, Bovie Medical, Megadyne as well as others.
In many monopolar electrosurgical applications, radio frequency (RF) power is delivered to the field of surgery by a surgical electrode or probe. The probe strongly focuses the RF current/power in a small contact area in the vicinity of the metallic tip of the probe and in the tissue in contact with it (often less than few square millimeters). As a result, the desirable effect of heating, coagulation, ablation, cutting etc. takes place in this small area. The probe is connected to the “output” of the electrical generator by an insulated wire which, in many cases, goes through the subject body. The return current conductor is connected to the “ground” or “return” terminal of the generator through a large area electrode placed on the surface of the subject body. This electrode collects the current induced in the subject body.
In electrosurgery, it is essential that the RF power be strongly focused in the immediate vicinity of the location where the desired procedure is performed. Not less important is a strong defocusing or dispersion of the electrical current beyond the target point of the surgery. This strongly dispersed current, or “return current”, should go through the subject body without harmful effects; for example, heating above a safe level can possibly lead to burns. Eventually all current is collected by the large area (on the order of few hundred square centimeters) electrode (i.e., the dispersive electrode) attached to the surface of the subject body and returned to the electrical generator by an insulated “ground” wire. Peak current density collected by the return electrode is affected by the current distribution over the area of that electrode. The distribution of the “return” current over the area of the dispersive electrode is affected, among other things, by the location of the surgical area with respect to the location of the dispersive electrode, the dispersive electrode area, and by the physical size of the subject's body between these areas. Many cases of the calculated non-uniform current density distribution under biomedical electrodes are described in the literature (e.g., Vessela Tz Krasteva et al., “Estimation of the current density distribution under electrodes for external defibrillation”, BioMedical Engineering Online Journal, 16 Dec. 2002). Subject's safety is achieved by dispersing the “return” current over the large surface area of the return electrode.
Accordingly, it is extremely difficult, but very important, to ensure that the current density distribution in the proximity of the dispersive electrode be as uniform (homogeneous) as possible. Since the return electrode is usually placed on the surface of subject's body, the cross section of the return current channel (i.e., the physical size of the subject's body) is sharply and abruptly decreased to the size of the dispersive electrode itself at the position where the electrode is attached to, and in contact with, the body. As a result, the dispersive electrode always compresses, or focuses, the return current in its vicinity. In all cases, unless special corrective measures are taken, the collected current distribution over the area of the return electrode is different from the desired uniform, smooth distribution. The result is that an extremely non-homogeneous distribution exists on the surface of the dispersive electrode, and in the proximity of the electrode below its surface (in the subject's body). Often current density near the outer edge of the dispersive electrode may be 10 times higher than at the center. In practice, this means that undesirable heating of the subject's body is strongly enhanced in the proximity of the outer edge of the dispersive electrode; this is often called the “edge effect”. The tendency of electrosurgical return current to cluster and generate heat in the vicinity of the corners and the outer edges of the return electrodes has been a long-standing design/safety problem that can lead to subject burns. Because of this inefficient current distribution, safety consideration have dictated that:    (a) Return electrodes must be much larger than necessary so as to provide homogeneous current distribution (i.e., the physical area of the electrode must be much larger than its “effective” area), and    (b) Most return electrodes must be positioned on the subject body with the long edge facing the surgical site to avoid burns.
Dispersive electrodes are also used for external defibrillation. Defibrillation of the heart is a widespread and well-established procedure for resuscitation of cardiac arrest victims. The most accessible approach for electrical cardiac therapy is via external electrodes, placed on selected locations on the surface of the thorax. The electrodes have a large surface area, and provide the high, and supposedly uniform, current density distribution in the heart needed for excitation of most myocardial cells, thus forcing them to return to normal rhythm. However, it has been reported that with conventional electrodes about 25% of the myocardium volume could be subject to current densities more than four times higher than the threshold density. Another aspect of the problem is the predominance of high current density along the perimeter of defibrillator electrodes applied on human skin (same edge effect discussed earlier). This can lead to unwanted damage and even severe skin burns or electroporation.
As current density is highly non-uniform across the return electrode, and is very high close to its edge, there is a risk of burns due to the tendency of the current and heat to cluster at the edges of the return electrode. Therefore, for safety reasons the pads are made much larger than needed. However, the larger the pad, the more difficult it is to place on subjects with limited muscle tissue, especially the elderly, babies and children. Suitable pad placement on burn victims and subjects with implants, excessive hair, scar tissue or skin problems has also proven to be difficult.
Accordingly, it is readily apparent that the art needs biomedical return electrodes capable of reducing the “clustering” of current at the edges of the electrode. The present invention solves the problems discussed above by providing a novel biomedical return electrode in which the current density distribution in the proximity of the dispersive electrode is much more uniform. In particular, the electrodes of the present invention are capable of altering and greatly improving the uniformity of current density profile in the proximity of the interface between the electrode and mammalian tissue. As a result, the chance for burns and other tissue damage to the skin as well as discomfort experienced by subject during or after usage is reduced.
In sum, the present invention provides a biomedical dispersive electrode which favorably redistributes the current in the subject's body in the proximity of the interface between the electrode and subject tissue, increases subject safety, and reduces the chance for burns and other tissue damage as well as discomfort experienced by subjects during or after usage. This new approach makes it possible to substantially reduce the size of the electrode, without compromising subject safety. Smaller size will improve the ease of use required in subject care environment.